Steel sheets for automobiles are required to be light from the viewpoint of improving fuel consumption and overcoming environmental problems, etc. Various high tensile strength steel sheets have, therefore, been developed for the purpose of using thin steel sheets.
Moreover, attention has also been paid to strengthening a steel sheet from the viewpoint of the safety of automobiles, and the future development strengthened steel sheet is anticipated.
The workability (plastic elongation) of steel sheets for automobiles is lowered as they are strengthened. Accordingly, securing both high tensile strength and good elongation is a problem to be solved in the development of high tensile strength steel sheets, and a great deal of research has been conducted in this area.
On the other hand, lubricants used for press working steel sheets have also been developed to avoid the partial constraint thereof during press working. The viscosity of the lubricant is increased to improve the oil film-forming ability during press working, and an extreme-pressure additive is used for preventing partial seizure.
On the other hand, steel sheets which are of higher grade and more diversified than those currently used are required as products become more accurate and complicated. Steel sheets are also required to have coating sharpness. There has been known a technique, to responds to this requirement, comprising forming fine recesses and protrusions on a roll using a laser and rolling a steel sheet with the roll to obtain a steel sheet excellent in coating sharpness, as disclosed, for example, by Japanese Unexamined Patent Publication (Kokai) No. 63-132701.
Even when the elongation of a high tensile strength steel sheet is obtained for the purpose of aiming at an improvement in the press workability, a decrease in the elongation caused by strengthening the steel sheet has heretofore been inevitable. As a result, an improvement in the workability cannot be expected. Accordingly, to improve the workability of the steel sheet, an improvement in the lubricating ability may be considered. However, when the viscosity of a lubricant oil is increased, there arise serious problems that the capacity of existing machines is lowered and that the cleanability in the latter step is lowered. As a result, there is a limitation on increasing the viscosity, and the viscosity cannot be greatly increased.
Moreover, since the action of an extreme-pressure additive is fundamentally of a reaction type, the addition thereof in a large amount influences the cleanability and coating properties of steel sheets.
Furthermore, the coating sharpness of a steel sheet is more improved when the surface of the steel sheet becomes closer to a mirror-finished surface. Various proposals have been made in the direction of decreasing the surface roughness thereof. However, when the surface roughness is decreased, defects thereon tend to become manifest in the steps of heat treating and plating. Moreover, surface defects are formed during piling and press working subsequently to shearing the steel sheet, and become quality defects. Accordingly, the surface roughness Ra of a steel sheet is currently made at least 0.75 .mu.m. An object of the present invention is to solve the recent problem of press workability associated with strengthening a steel sheet and provide a steel sheet of high quality corresponding to the coating sharpness which is a quality thereof.